1. Field of the Invention
The present invention relates in general to a feedback compensating apparatus used with a controller for a working machine (e.g., a metal working machine such as a grinding machine), which apparatus is adapted to adjust the working condition of the working machine by feeding back to the controller a compensating signal or data obtained during and/or after a working operation on the workpiece, so that the controller controls the working machine so as to improve the dimensional accuracy of the workpiece processed by the machine. More particularly, the present invention is concerned with techniques for improving the accuracy of adjustment of the working condition by the compensating apparatus.
2. Discussion of the Prior Art
For assuring high dimensional accuracy of processed portions of a workpiece after the workpiece is machined, ground or otherwise processed, there are available various feedback control methods such as an in-process control method, a post-process control method, and a hybrid control method These control methods are practiced, for example, to finish inner cylindrical surfaces of cylinder bores formed in cylinder blocks of an engine of a motor vehicle, or outer cylindrical surfaces of a crankshaft of such engine, with high precision, so that the dimension such as the inside diameter of the finished cylinder bores or outside diameter of ground journals of the crankshaft is as close to the nominal value as possible.
The in-process control employs an in-process measuring device for measuring the dimension of the working portion of the workpiece during a working operation on the workpiece, so that the working operation is terminated when the measured dimension coincides with the nominal value.
Usually, the in-process control is based on an assumption that the desired or nominal dimension is obtained when the actually measured dimension reaches a reference value corresponding to the nominal dimension. To this end, the dimension of the working portion of the workpiece is continuously measured, and the measured dimension is compared with the reference value, to determine the moment when the actually measured dimension becomes equal to the reference value.
The post-process control employs a post-process measuring device for measuring the dimension of the working portion of the workpiece after the working portion has been processed or finished. A difference of the actually measured dimension from the nominal value, namely, a dimensional error of the finished workpiece is fed back to the controller for the working machine, so that the working data such as numerical control data (NC data) subsequently used by the machine for effecting the working operation on the following workpiece or workpieces are suitably adjusted so as to eliminate or reduce the dimensional error.
The post-process measuring device does not necessarily act on the workpiece or its working portion immediately after the appropriate working operation, but may operate in a mode in which the measurement is effected a given length of time after the working operation, during which the other working portions or other workpieces are machined or otherwise processed.
The hybrid control, which is a combination of the in-process and post-process control methods, is adapted to feed back the dimensional error of the processed workpieces obtained by the measurement by the post-process measuring device, to compensate the reference value used in the in-process control, for zeroing the post-process dimensional error.
However, the known control methods indicated above and the apparatus to practice these known methods suffer from a problem that the control logics, rules and programs used in the known systems do not permit sufficiently high accuracy of adjustment of the working condition of the working machine. One presumed reason for the insufficient accuracy in the prior art lies in the mere use of the dimensional error per se to adjust the working condition, whereas various other factors or parameters affect the actual dimensions of the working portions of the workpieces and should be taken into account for improved accuracy of compensation of the working condition to eliminate the dimensional error.